Numitrons. They display eight different programmed
patterns, most based on a one minute repeat cycle.
The Numitrons, on the other hand, do not lend
themselves at all to multiplexing, and therefore a BCD
decoder (with latch) was used to drive each tube. I’ll
explain more about the operation of those later on. The
schematic and board were created in Eagle, and the files
are available for download at the article link in native
Eagle format as well as the Gerber RS274x format.
Programming of the PIC is done through the ICSP
connector located on the board. I use the PICkittm3
combined with MPLAB IPE (integrated programming
environment); refer to Figure 5. Other options are
available for programming, although programming SMD
devices off-board often requires a special adapter. The
main system crystal frequency is 4 MHz. The clock (time)
crystal is a small 32 kHz watch crystal. NOTE: This is a
critical part for clock accuracy! Please resist using an
unknown or salvaged part here.
The two 12 pF load capacitors (Figure 6) determine
its frequency accuracy, and must be matched with
the part. Accuracy of one second per day or less
is possible if the crystal and load capacitors are
properly matched. If you use a different crystal
than listed, be sure to change the load capacitor
values to the specification of your part.
A 0.1 µF ceramic capacitor is placed close to the PIC
(as well as the other chips) to provide noise decoupling.
After all, there is a lot of heavy duty switching happening
on this board. The reset button is not debounced (not
needed), and for all intents and purposes may be left off
the board all together. Simply disconnecting and
reconnecting the power will do the same, and prevent
non-intentional reset when in use.
The Power Supply
The power supply is fairly simple, using a switch mode
buck converter to provide the five volts required for the
clock (Figure 7).
Though I considered a linear regulator, the part
became hotter than I was comfortable with while testing
in a circuit like this. Additionally, a switch mode supply
allows for more choices when it comes to wall wart style
power supplies (Figure 8). The greatest current
requirement is during the start-up flash cycle, where all six
Numitrons are lit for a second at the time; 7x6 segments
at about 23 mA = 966 mA total. Maximum current
28 September 2016
■ FIGURE 5. The PICkit3 connected to the Numitron board
■ FIGURE 6. The clock timing crystal
installed on the PCB with the surface-mount
noise decoupling capacitors.
■ FIGURE 7. Schematic of the
power supply section
of the project.